The addition of silver affects the deformation mechanism of a twinning-induced plasticity steel: Potential for thinner degradable stents

Abstract

While Fe-based alloys have already been reported to possess all mechanical properties required for vascular stenting, their relatively low degradation rate in vivo still constitutes their main bottleneck. The inflammatory reaction generated by a stent is inversely proportional to its mass. Therefore, the tendency in stenting is to lower the section so to reduce the inflammatory reaction. Twinning-induced plasticity steels (TWIP) possess excellent mechanical properties for envisaging the next generation of thinner degradable cardiovascular stents. To accelerate the degradation, the addition of noble elements was proposed, aimed at promoting corrosion by galvanic coupling. In this context, silver was reported to generally increase the degradation rate. However, its impact on the deformation mechanism of TWIP steels has not been reported yet. Results show that the use of Ag significantly reduces the ductility without altering the strength of the material. Furthermore, the presence of Ag was found to promote a different deformation texture, thus stimulating the formation of mechanical martensite. Since a stent works in the deformed state, understanding the microstructure and texture resulting from plastic deformation can effectively help to forecast the degradation mechanisms taking place during implantation and the expected degradation time. Moreover, knowing the deformed microstructure allows to understand the formability of very small tubes, as precursors of the next generation of thin section degradable stents. STATEMENT OF SIGNIFICANCE: Commercial degradable magnesium stents are limited from their relatively big structure size. Twinning-induced plasticity steels possess outstanding mechanical properties, but their degradation time goes beyond the timeframe expected from clinics. The inclusion of noble Ag particles, which favor galvanic coupling, is known to promote corrosion and solve this limitation. However, it is necessary to understand the impact that Ag has on the deformation microstructure and on the mechanical properties. The addition of Ag reduces the ductility of a twinning-induced plasticity steel because of a different deformation microstructure. Since a stent works in a deformed state inside an artery, understanding the microstructural evolution after plastic deformation allows to better predict the device performances during service life.

Keywords: Biodegradable metals; Deformation texture; Silver; Stents; Twinning-induced plasticity steel.